The male/female ratio of fetal deaths and births in Japan

doi:10.1016/S0140-6736(00)02637-4

The Lancet

Volume 356, Issue 9231, 26 August 2000, Pages 738-739

https://doi.org/10.1016/S0140-6736(00)02637-4 Get rights and content

Summary

The decline in the male/female ratio of births has not yet been explained. In Japan, the male/female ratio of fetal deaths has been increasing since the 1970s, reaching over 2·0 in 1996. The increased male/female ratio of births could explain the decrease in the male/female ratio of births during the past two decades. This trend could suggest the increasing prenatal vulnerability of the male fetus especially at earlier stages of gestation.

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According to the literature, SSR has shown notable heterogeneity and a common pattern was observed in some developed countries: the SSR first showed increasing trends during the first half of the twentieth century, and then the trend decreased during the second half (Fellman, 2015). A similar secular trend increase/decrease was also observed in Japan (Mizuno, 2000). In this context, our findings suggest that the reduced lead exposure may be involved in the above recent decrease in SSR.
A decline in the proportion of male births (secondary sex ratio, SSR) has been seen in several countries including Japan in recent years. Although previous studies have reported that the SSR is affected by exposure to chemical substances such as dioxins and polychlorinated biphenyls, as well as heavy metals such as methylmercury, the effects of lead exposure on the SSR have been little studied. The aim of this study was to determine the association between maternal lead exposure and SSR. In a large-scale nationwide birth cohort study, maternal blood lead level (BLL) was determined using whole blood from the second or third trimester of pregnancy. The association between SSR and maternal BLL was estimated using multivariable logistic models. Binomial distribution was applied to examine the differences in SSR by dividing the participants into five groups based on BLL. The primary outcome was SSR, and the child sex was obtained from the medical record transcripts. Of 104,062 fetal records, 85,171 were examined for analysis. The median maternal BLL was 5.85 ng/g (5th–95th percentile 3.45–10.6 ng/g). The overall proportion of males among participating infants was 0.512. In logistic regression models adjusted for covariates, the analysis revealed an increased odds ratio for SSR with higher blood lead concentrations [Group 2: adjusted OR 1.082, 95% confidence interval 1.037 to 1.129, Group 3: 1.122, 1.074 to 1.171, Group 4: 1.214, 1.163 to 1.268, Group 5: 1.279, 1.224 to 1.336]. Compared to the general birth probability in Japan, the group with low BLL had a lower SSR and the group with high BLL had a higher SSR. Higher maternal lead exposures during pregnancy were associated with increased SSR. Further investigations including assessment of paternal lead exposure are necessary to understand the association between lead exposure and SSR.
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The resulting live births would generate the natural (secondary) sex ratio, i.e., the odds of a newborn's being male. Following the work of Trivers and Willard (1973), however, a number of studies suggest that in utero shocks (of different nature, including terrorism, pollution, maternal fasting, earthquake, cold waves) may increase fetal loss, and more specifically may reduce the number of boys relative to girls at birth, underlying the need to better grasp the mechanisms at play during pregnancy to correctly interpret outcomes emerging later in life (Shettles, 1961; Mizuno, 2000; Kraemer, 2000; Catalano et al., 2006; Roseboom et al., 2001; Almond et al., 2008; Hernández-Julián et al., 2014; Eriksson et al., 2010; Black et al., 2016; Fukuda et al., 1998; Torche and Kleinhaus, 2012; Catalano et al., 2006, 2008; Sanders and Stoecker, 2011; Almond and Mazumder, 2011). Two separate mechanisms intertwine as a result of in utero shocks, both resulting in a reduction of the (secondary) sex ratio: the culling (or selection) and the scarring effects (Valente, 2015; Catalano and Bruckner, 2006).4
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Citation Excerpt :
These data were used to obtain detailed care information on term SGA infants who died in the perinatal period [19]. The following factors were analyzed: maternal age ( < 20, 20–24 and ≥35 years), nulliparity, maternal non-Western ethnicity/race (African, South Asian, Moroccan/Turkish, and other non-Western), low socio-economic status (SES) (<p25), deprived neighborhood, hypertensive disorders (hypertension, (pre)eclampsia, HELLP), male gender and gestational age (37, 38, 39, 41 and 42 completed weeks) at delivery [24–26]. The classification of ethnicity was performed by the healthcare professional on the basis of race and country of birth.
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Research LettersThe male/female ratio of fetal deaths and births in Japan

Summary

Lancet

Reduced ratio of male to female births in several industrial countries

JAMA

Research Letters
The male/female ratio of fetal deaths and births in Japan